Trimble and Wellington (1980) found that female A. togoi prefer to oviposit on water that has held conspecific larvae and pupae to water that has not. Repeated.
THE EFFECT OF CONSPECIFICS ON OVIPOSITION SITE SELECTION AND OVIPOSITION BEHAVIOUR IN AEDES T O W 1 (THEOBOLD) (DIPTERA: CULICIDAE)
DAVID Y. ONYABE and BERNARD D. ROITBERG~ Centre for Pest Management, and Behavioural Ecology Research Group, Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6
Abstract
The Canadian Entomologist 129: 1173 - 1176 (1997)
Two opposing hypotheses are tested regarding the choice of oviposition sites by female Aedes togoi (Theobold) mosquitoes: (i) conspecific avoidance-females discriminate against sites harboring conspecifics to reduce intraspecific competition for their offspring and (ii) conspecific attraction-females prefer sites with conspecifics because their presence indicates suitable conditions for larvae. Under laboratory conditions, A. togoi females laid many more eggs on rearing water (LRW) containing conspecific larvae, their waste, bacteria, and food supplements than on rearing water alone (RM). In another experiment, females showed an oviposition preference for LRW sites that were devoid of A. togoi eggs compared with those harboring 50 (0.3 eggs/mL) conspecific eggs. Further, it was discovered that females laid their eggs at several sites rather than at a single suitable site. Possible reasons for such choices are discussed. Onyabe, D.Y., et B.D. Roitberg. 1997. Effets des individus conspecifiques sur le choix d'un site de ponte et sur le comportement de ponte chez Aedes togoi (Theobold) (Diptera : Culicidae). The Canadian Entomologist 129: 1173-1 176
Nous avons CprouvC deux hypothkses contraires au sujet du choix d'un site de ponte chez les femelles d'Aedes togoi (Theobold) : (i) la fuite des individus conspCcifiquesles femelles Cvitent les sites occupCs par des conspkifiques de f a ~ o n8 rauire la compktition intraspkcifique chez leurs rejetons et (ii) I'attirance pour les individus conspCcifiquesles femelles prkferent les sites dCj8 occupCs par des conspCcifiques parce que leur prCsence est indicatrice de conditions favorables pour les larves. Dans des conmtions de laboratoire, des femelles #A. togoi ont pondu beaucoup plus d'oeufs dans de l'eau d'Clevage (LRW) contenant des larves conspCcifiques, leurs excrements, leurs bactCries et leurs supplCments alimentaires que dans de l'eau d'Clevage pure (RM). Au cours d'une autre expCrience, les femelles ont manifest6 des prCfCrences pour les sites LRW ne contenant pas d'oeufs d'A. togoi plutbt que pour les sites contenant dCj8 50 (0,3 oeufs/mL) oeufs conspCcifiques. De plus, nous avons constat6 que les femelles pondaient leurs oeufs i plusieurs endroits plutbt qu'8 un seul site favorable. Nous examinons les raisons possibles de ces comportements multiples, propres B Cquilibrer les risques. [Traduit par la RCdaction]
Introduction The decision of where and how many offspring to deposit in breeding sites is critical for the fitness of juveniles of animals like mosquitoes that are unable to move to more suitable habitats if the local site is unfavourable. All else being equal (Charnov and Skinner 1985), therefore, natural selection should favour females that select habitats which best support the development of their offspring. We investigated oviposition site selection in Aedes togoi (Theobald). This mosquito is found in various parts of Asia, but also occurs on the Pacific Coast of North
' Author to whom all correspondence should be addressed.
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America (Trimble and Wellington 1979; Belton 1980). In North America, preadult development is restricted to shallow rock pools situated above the high-tide level (Belton 1980). Trimble and Wellington (1980) found that female A. togoi prefer to oviposit on water that has held conspecific larvae and pupae to water that has not. Repeated utilization of the same pools may lead to dense larval populations in such pools. Given that increasing larval density may result in higher larval mortality, protracted larval development, and reduced adult size (e.g. Reisen 1975), how might female oviposition behaviour possibly mitigate these consequences? Females that can detect and oviposit less frequently in sites already occupied by conspecifics, potential intraspecific competitors, should presumably be favoured by natural selection (Blaustein and Kotler 1993). Conversely, the presence of conspecifics may indicate to a female that the habitat can support offspring (Blaustein and Kotler 1993). Alternatively, regardless of the presence or absence of conspecifics, females may distribute eggs per batch into two or more oviposition sites (rather than depositing all eggs in one pool) to improve the chances that at least some of a female's offspring will survive. Materials and Methods We tested these opposing hypotheses in two experiments. Experiment 1 examined habitat selection, but especially egg distribution, when single females were provided with oviposition sites containing water that previously contained conspecific larvae and water that had not (Trimble and Wellington 1980). In experiment 2, single caged females were presented with a choice between oviposition cups containing conspecific eggs and cups with no eggs. In both experiments, the egg-distribution pattern was assessed by providing each female with six cups (three of each of the respective treatments). The mosquitoes in this study were obtained from the eggs of A. togoi collected from the field (Lighthouse Park, West Vancouver, BC) as larvae or pupae, and reared as described by Trimble and Wellington (1979). Emerging females were kept with males for mating and were blood fed according to Trimble and Wellington (1979, 1980). In experiment 1, each of 30 females was isolated in a 30 x 30 x 30 cm cage and provided with six plastic cups (height 8.5 cm, bottom diameter 4.5 cm, and top diameter 7 cm) for oviposition. Three cups contained 150 mL of freshly prepared rearing medium (RM), a 1:1 sea water - distilled water mixture filtered through No. 1 whatman'" filter paper (Trimble and Wellington 1979). The other three cups contained 150 mL of filtered larval rearing water (LRW) obtained from several batches of larvae that were reared as described by Trimble and Wellington (1979). Like RM, LRW was derived from a 1:l sea water - distilled water mixture (Trimble and Wellington 1979) except that 50 fourth-instar larvae had been reared in it for 2 days, thus it contained larval wastes, bacteria, and food supplements (~etramin'"). RM never previously held larvae nor contained food. The cups were placed, three each on parallel sides of the cages used; however, the exact position of cups presented to each female was determined by random draw. Each female was provided with 15% sucrose solution in the cage and each was allowed to lay only one batch of eggs (4-5 days later). In experiment 2, to investigate oviposition response to the presence of conspecific eggs, each of 24 females was offered six oviposition cups, each containing 150 mL of filtered LRW. However, three of the six cups, in addition to LRW, each contained 50 freshly laid eggs (this being the average number of eggs laid per female per cup in experiment 1). The conditions maintained and procedures followed in this experiment were as outlined for experiment 1.
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TABLE1. Selection of oviposition site by female Aedes togoi betweealarval rearing water (LRW) and rearing medium (RM) (experiment 1, n = 30), andhetween cups containing LRW with and without conspecific eggs (experiment 2, n = 24) Eggs deposited
Cups accepted
Experiment 1 LRW RM t
P Experiment 2 Cups with eggs Cups without eggs t
P
In both experiments, the following measurements were made: (i) total number of eggs laid per treatment by each female, and (ii) proportion of cups accepted and rejected per treatment by each female. Statistical comparisons of data from each female were made with the paired-sample t test. However, before perfonping the paired-sample t test, the proportion of cups accepted and rejected was arcsine transformed so that the data would better approximate a normal distribution (Zar 1984).
Results and Discussion The results of experiment 1 are summarized in Table 1. Females laid over 11 times as many eggs on LRW as they did on RM and oviposited in signikmtly more LRW cups than in RM cups. Only 10 of 30 females in this experiment laid anygaggs on RM, whereas all 30 deposited some eggs on LRW. Twenty-six females deposited eggs in at least two of the six cups provided, whereas only four laid all eggs in one cup. The results of experiment 2 are in part equivocal. Females accepted twice as many cups containing no conspecific eggs as they did cups with conspecific eggs (Table 1). Although more eggs were laid in cups that contained no conspecific eggs than in cups with conspecific eggs, the difference was not statistically significant (Table 1). Twenty of 24 females in the study laid eggs in two or,more of the six cups provided, whereas only four laid all their eggs in one cup. Fmthermore, 10 of 24 females did not oviposit in any cups containing conspecific eggs, compared with only one female that laid no eggs in cups with no conspecific eggs. In agreement with Trimble and Wellington (1980), we observed in experiment 1 that female A. togoi seemingly preferred to lay eggs on water that previously held conspecifics, most likely because it contained food supplements,,but possibly because of larval-associated wastes or bacterial metabolites in the water (Trimble and Wellington 1980). In contrast, female Anopheles gambiae Giles do not show a preference for water that has held conspecific larvae over water that has not (McCrae 1984). Taken together, the results of experiment 2, although partly equivocal, probably mean that caged females can detect conspecific eggs, and will oviposit more frequently in sites with no conspecific eggs. Whenever they accept sites with conspecifics eggs, however, they lay just as many eggs as they would in their absence. In contrast, in response to an egg-associated pheromone (Osgood 1971; Laurence and
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Pickett 1982), some species of Culex preferentially oviposit in sites containing egg rafts of their own or a related species (Osgood 1971; Bruno and Laurence 1979). Judging from the distribution of eggs among cups in both experiments, it seems most A. togoi literally do not put their eggs per batch in one "basket" (Seger and Brockmann 1987). Depositing eggs in two or more sites, if this observation holds true under field conditions, will distribute offspring per female among several pools. The close proximity (sometimes less than 1 m) of rock pools in the field, and the fact that A. togoi usually lay single eggs (rather than in rafts, e.g. Culex), would facilitate the distribution of eggs among several pools. The results of experiment 1 suggest that females will likely distribute their eggs mostly among pools that have held conspecific larvae or contain food or bacteria. The quality of some selected sites may not be as suitable as others; however, on average, such bet-hedging would likely improve the chances that at least some of a female's offspring will survive if, for example, some pools are ephemeral. We suggest that caged A. togoi seemingly can detect the presence of conspecific eggs, will select such sites less frequently for oviposition, and like Aedes aegypti L.(Reiter et al. 1995), each female tends to distribute eggs per batch among several pools.
Acknowledgments We thank P. Belton and G. Anderson for discussions, and R. Balshaw for statistical advice. This work was supported by a Natural Sciences and Engineering Research Council of Canada grant to B.D.R.
References Belton, P. 1980. The first record of Aedes togoi (Theo.) in the United States-aboriginal or ferry passenger? Mosquito News 40: 624-626. Blaustein, L., and B.P. Kotler. 1993. Oviposition habitat selection by the mosquito, Culiseta longiareolata: effects of conspecifics, food and green toad tadpoles. Ecological Entomology 18: 104-108. Bruno, D.W., and B.R. Laurence. 1979. The influence of the apical droplet of Culex egg rafts on oviposition of Culex pipiens fatigans (Diptera: Culicidae). Journal of Medical Entomology 16: 300-305. Charnov, E.L., and S.W. Skinner. 1985. Complementary approaches to the understanding of parasitoid oviposition decisions. Environmenrnl Entomology 14: 383-391. Laurence, B.R., and J.A. Pickett. 1982. Erythro-6-acetoxy-5-hexadecanolide, the major component of a mosquito oviposition attractant pheromone. Journal of the Chemical Society Chemical Communications 1: 59-60. McCrae, A.W.R. 1984. Oviposition by African malaria vector mosquitoes. 11. Effects of site tone, water type, and conspecific immatures on target selection by freshwater Anopheles gambiae Giles, sensu lato. Annals of Tropical Medicine and Parasitology 78: 307-318. Osgood, C.E. 1971. An oviposition pheromone associated with the egg rafts of Culex tarsalis. Journal of Economic Entomology 64: 1038-1041. Reisen, W.K. 1975. Intraspecific competition in Anopheles stephensi Liston. Mosquito News 35: 473-482. Reiter, P., M.A. Amador, R.A. Anderson, and G.G. Clark. 1995. Short report: Dispersal of Aedes aegypti in an urban area after blood feeding as demonstrated by rubidium-marked eggs. American Journal of Tropical Medicine and Hygiene 52: 177-179. Seger, J., and H.J. Brockmann. 1987. What is bet-hedging? pp. 182-211 in Harvey, P.H., and L. Partridge (Eds.), Oxford Surveys in Evolutionary Biology 4. Oxford University Press, Oxford,England. Trimble, R.M., and W.G. Wellington. 1979. Laboratory colonization of North American Aedes togoi. Mosquito News 39: 18-20. 1980. Oviposition stimulant associated with fourth-instar larvae of Aedes togoi (Diptera: Culicidae). Journal of Medical Entomology 17:509-514. Zar, J.H. 1984. Biostatistical Analysis. Prentice-Hall Inc., Engelwood Cliffs, NJ. (Date received: 19 September 1996; date accepted: 18 June 1997)
